1. Field of Invention
The present invention relates to a voltage sensing method, which can be applied to a control circuit of a power supply which converts a voltage by a transformer, such as a photoflash charger control circuit.
2. Description of Related Art
Power supplies which convert a voltage by a transformer are applied in many applications. One of the applications is chargers such as a photoflash charger. The basic structure of such charger is shown in the upper part of FIG. 1. A capacitor Cout at the output terminal Vout is charged from an input terminal Vin via a transformer. The charging time is controlled by a power switch 21 in a charger control circuit 20. The charger control circuit 20 is typically integrated as an integrated circuit (IC). In the prior art shown in FIG. 1, the power switch 21 is controlled by a switch control circuit 25, and the switch control circuit 25 determines whether to enable the power switch 21 according to a voltage detection at the secondary side of a transformer 10. More specifically, a feedback voltage is obtained through resistors R1 and R2 by voltage division. A comparator 23 compares the feedback voltage with a reference voltage Vref; then the switch control circuit 25 determines whether to enable the power switch 21 according to the result of the comparison.
The drawback of the prior art mentioned above is that resistors R1 and R2 (particularly, R1) need to sustain high voltage, because the dividend voltage is obtained from the secondary side which is the high voltage side. Some prior art proposes to divide the resistor R1 to two resistors, but the fact that they need to sustain high voltage remains the same.
U.S. Pat. Nos. 7,292,005; 6,636,021; and 6,518,733 disclose another type of approach as shown in FIG. 2. This approach obtains a signal from the primary side, and compares the signal with a reference signal Vref in the comparator 23. The result of the comparison is transmitted to the switch control circuit 25, which determines whether to enable a power switch 22 based on the comparison. In this prior art, the power switch is implemented by a bipolar junction transistor 22, but the basic principle remains the same. This prior art also discloses a one-shot circuit 24 for masking a switching ringing.
One drawback of the second prior art is that, even though the feedback voltage is not obtained from the secondary side, and the resistor specification for sustaining high voltage can be relatively lower, but it requires two external resistors R3 and R4.
With respect to switching ringing, U.S. Patent Publication No. 2006/0250824 discloses a method to filter such noise by a low-pass filter circuit.
Even though the second and third prior art avoid the problem in the first approach which obtains the feedback signal from the secondary side, i.e., the requirement of devices capable of sustaining high voltage, they still have one common drawback as described below. The transformer employed in the charger may have different turn ratios in different applications. The prior art power supply control circuit can not adjust its output voltage detection and setting in correspondence with different turn ratios (the detection determines where the output voltage is balanced at, and therefore the adjustment of the detection can be regarded as the adjustment on the output voltage setting). If the transformer turn ratio is different, it is necessary to modify the internal circuitry of the charger control circuit, and re-produce a different integrated circuit to cope with it. In other words, the same integrated circuit can only be applied to one single application.
In view of the above drawbacks, it is desired to provide a power supply control circuit and a method for sensing voltage in a power supply control circuit, which do not require a device sustaining high voltage, and furthermore the output voltage detection and setting can be flexibly adjusted in correspondence to the transformer turn ratio.